Multiple myeloma (MM) is a neoplasm of bone marrow (BM) resident plasma cells (PC) that comprises 20% of all hematologic malignancies, and despite new chemotherapeutic agents remains incurable. Myeloma cells are critically dependent on interactions with BM stromal niches for their survival, just like their normal counterparts the BM-resident long-lived PC (LLPC). It is also becoming evident that these interactions play a major role in MM resistance to chemotherapy, the primary cause of treatment failure in this cancer. Despite their central importance however, the specific molecular and cellular components involved in these interactions remain poorly characterized. Although it is primarily characterized as the prototype T cell costimulatory receptor involved in T cell activatio and survival, CD28 is also expressed on normal and malignant PC - and in myeloma its overexpression correlates significantly with poor prognosis, disease progression and shorter survival. Over the last funding cycle, we have shown in patient gene expression profiles that CD28 overexpression is associated with disease progression and poor prognosis patient subgroups, and that activation of CD28 on MM cells protects against chemotherapy-induced death. We have now found that blocking CD28 activation leads to MM cell death in vitro and in vivo, and sensitizes the myeloma to chemotherapy. In addition, we have made the novel observation that PC-intrinsic CD28 function also plays an essential role in sustaining the survival of normal BM LLPC. Thus it appears that CD28 regulates fundamental aspects of normal PC biology that are conserved in myeloma biology due to their essential role in survival. We have subsequently determined that the Vav pathway downstream of CD28 activation transduces CD28's pro-survival signal in BM LLPC. This pathway upregulates expression of the transcriptional regulator BLIMP-1, the master regulator of normal PC identity - but whose role in MM biology has been largely unexplored. We now hypothesize that the CD28 Vav pathway regulates a central transcriptional nexus that is required to maintain PC identity, and that this identity is necessary for MM survival and resistance to chemotherapy. The overall goal of this proposal is to define the signaling pathways and molecular targets downstream of CD28 activation that support myeloma cell survival. This understanding is likely to lead to the identification of new therapeutic targets and development of novel treatment strategies for MM, especially chemotherapy resistant disease. The Specific Aims are: 1). Define the signaling pathways downstream of CD28 activation that intrinsically regulate myeloma cell survival, 2). Define the molecular targets of CD28 Vav signaling that regulate myeloma cell survival, and 3). Determine the anti-myeloma efficacy of blocking CD28 activation in vivo.